This invention relates to heat exchangers and their methods of use.
Heat exchangers provide a structure in which thermal energy can be transferred from a hot medium to a cold medium. In particular, a heat exchanger may provide a structure in which a flame or hot gases heat, melt, or drive chemical reactions in particulate material. For example, in glass manufacture, a furnace melts glass batch feedstock and cullet (recycled glass) using a flame formed from natural gas, oil, or coal, and preheated combustion air. Radiation from the flame and furnace enclosure transfer heat to the melt. In some cases, a separate heat exchanger preheats the glass batch and cullet using hot exhaust gases from the furnace.
In another example, a calciner furnace heats limestone (CaCO.sub.3) to drive off carbon dioxide (CO.sub.2) and calcine the limestone into lime (CaO). One type of calciner includes a pair of connected silos in which natural gas is burned between the silos. During its use, limestone is fed into each of the silos. Then, air is drawn into one of the silos and passes through the natural gas flame, which heats the air. The heated air then passes through the second silo and transfers the heat to the limestone in that silo to drive the calcination reaction. After a time, the direction of the air flow between silos is reversed to heat the limestone in the first silo.
A rotary kiln can also be used as a calciner. A rotary kiln includes a large rotating pipe oriented at a small angle from the ground. Particulate material flows through the pipe from top to bottom and is heated by counter flowing hot gases. Rotation of the pipe mixes and breaks up the particulate flow to provide even heating. In addition, the rotary kiln can include chains hanging within the pipe to further break up the particulate flow and to transfer heat to the particulate material.
In all cases, the design of a heat exchanger attempts to optimize the efficiency of the thermal transfer process. The sizes of the particulate material can affect this efficiency. For example, in the connected-silo calciner, the spaces between the chunks of limestone fed into the silos must be sufficiently large to provide a passage for the air flow with an acceptable pressure drop. Also, in the rotary kiln, small particulates can "blanket" larger particulates and prevent heat transfer to these larger particulates. Another concern is the loss of small particulates to exhaust gases from the heat exchanger.